This invention relates to the field of communications, and, in particular, to optical frequency modulation devices useable in satellite communications systems.
Orbiting satellites are an important aspect of modern communication systems. Originally used for xe2x80x9csingle-bouncexe2x80x9d communication, with a signal going up from one place on the surface of the earth and coming down in another, communication satellites are now being used to form complex networks in space, with each satellite in the network being able to communicate with many of the other satellites. Optical intersatellite links, with their high directionality, high energy efficiency, and tremendous information bandwidth, allow satellites to talk to one another, and to transmit a much larger amount of information.
Many satellite and terrestrial optical communication systems require transmission of analog optical signals. One known way to meet this transmission need is by employing amplitude modulation (AM) of an optical carrier. This approach, however, suffers from poor signal-to-noise ratio (SNR).
It is also well known that broadband modulation schemes, which utilize higher bandwidth than that of the transmitted information, may improve the SNR over that achieved with AM. One such technique is frequency modulation (FM). It is well known that the SNR of an FM system may be improved dramatically by using a higher frequency swing FFM than the bandwidth of the transmitted information xcex94f, as described by in H. S. Black""s xe2x80x9cModulation Theoryxe2x80x9d, published by D. Van Nostrand (1953), wherein:
SNRFMxe2x88x9dSNRAM(fFM/xcex94f)2,xe2x80x83xe2x80x83(1)
where SNRAM is the signal-to-noise ratio of an AM communication system with identical optical power.
It is also known that FM optical signals can be obtained by modulating the current of a semiconductor laser. This technique, however, suffers from simultaneous amplitude modulation, and it provides a very limited frequency swing fAM less than 20 GigaHertz (GHz).
In order to realize SNR advantages of optical FM, the FM receiver must include a limiter that eliminates amplitude noise of the received optical signal without affecting its frequency contents. Such all-optical limiters can be easily made for pulsed signals, e.g., based upon non-linear optical loop mirrors (NOLM), such as described in Wong et al. in Optics Letters, Vol. 22, 1997, p. 1150, or on SPM as described in the Mamyshev article, xe2x80x9cAll-optical Data Regeneration Based on Self-Phase Modulation Effectxe2x80x9d, ECOC98, p. 475. These techniques, however, are hard to implement for continuous optical signals.
Therefore, to better help realize practical optical inter-satellite links, there exists a need for an effective FM system and method for analog optical communication. The present invention provides a solution to meet such need.
In accordance with the present invention an apparatus and method of optical frequency modulation is provided based upon spectral broadening of optical pulses via self phase modulation (SPM) followed by frequency selection by a fast tunable Fabry-Perot filter formed by two Distributed Bragg Reflectors (DBRs).
For realizing the SNR advantages of FM, an optical communication system needs an optical limiter in its receiver that equalizes the amplitude of a signal without changing its frequency. In a high bandwidth communication system, it is highly desirable to do limiting in the optical domain because of electronic speed limitations. While optical pulse reshaping and limiting based on SPM effect in optical fibers followed by spectral filtering has been described in the Mamyshev article, in accordance with the present invention this technique is combined with a tunable DBR filter to achieve FM pulses for optical communication.
The proposed system benefits from its pulsed format that allows the use of NOLM, such as described in the aforementioned Wong et al. article, or other optical regeneration techniques, such as delineated in the Mamyshev article, for optical limiting, which is not possible in a continuous wave FM system.
The advantages of the inventive approach described hereinbelow include: (1) a large FM swing of several hundred GHz, which is important for improving SNR, and (2) compatibility with optical limiting.
Therefore, in accordance with a preferred embodiment of the present invention a method and apparatus for pulse frequency modulation for analog optical communication is provided. A train of optical pulses is generated. The spectrum of the optical pulses in the train of optical pulses can be broadened to provide a train of broad spectrum optical pulses. The broadening can be provided by self-phase modulation. Alternatively, broad spectrum optical pulses can be provided by merely having the optical pulse duration be shorter than 1 ps. A desired optical frequency slice from the broad spectrum optical pulses is selected by a tunable Fabry-Perot filter. The tunable Fabry-Perot filter has a pair of Distributed Bragg Reflectors separated by an electro-refractive section. The electro-refractive section has tuning electrodes for applying transverse electric fields to the electro-refractive section, corresponding to an analog waveform being applied to the tuning electrodes, to provide a pulse-frequency modulated train of optical pulses.
Recovery of the analog waveform from the pulse-frequency modulated train of optical pulses is also provided. The pulse-frequency modulated train of optical pulses is spilt into a first optical beam and a second optical beam. A first photodetector is provided, the first photodetector providing a first current responsive to the first optical beam input thereon. The first photodetector has a first photodetector spectral response and is biased such that the first current is in a first direction. A second photodetector is also provided, the second photodetector providing a second current responsive to the second optical beam input thereon. The second photodetector has a second photodetector spectral response and is biased such that the second current is in the first direction. An input of a transimpedance amplifier is coupled to an output of the first photodetector and to an input of the second photodetector to provide an output of the transimpedance amplifier proportional to the difference between the first current and the second current. A first optical filter is provided to receive the first optical beam prior to incidence upon the first photodetector and a second optical filter is provided to receive the second optical beam prior to incidence upon the second photodetector. The first photodetector spectral response and the second photodetector spectral response are each broader than respective passbands of the first optical filter and the second optical filter to provide photocurrent vs. optical frequency characteristics determined by the respective first optical filter and the second optical filter.